In the fixed vibration absorber prior art the absorbers are basically fixed frequency absorbers which are capable of absorbing vibration over a relatively small range of frequency of the principal excitation source. Typical of these absorbers are the swastika-type absorber shown in U.S. Pat. No. 3,005,520 to Mard and the battery absorber presently used in helicopters, which is basically a spring mounted weight and generally of the type disclosed in a U.S. Patent Application Ser. No. 913,720 entitled Leaf Spring Tuned Vibration Absorber With Improved Weight Efficiency by John Marshall II and filed on June 7, 1978, now U.S. Pat. No. 4,230,291. These prior art absorbers are fixed frequency absorbers which are capable of absorbing vibrations over a relatively small range of rotor RPM. In addition, they are generally heavy, create substantial friction, and have bearings which are susceptible to wear.
Bifilar-type vibration absorbers have conventionally been used solely on rotating mechanisms, such as crankshafts of automobiles and aircraft engines and on helicopter rotors shown in Paul and Mard U.S. Pat. No. 3,540,809. In these installations, the centrifugal force generated by rotation of the mechanism involved is necessary for the operation of the bifilar-type vibration absorber. In a fixed position vibration absorber of the type sought in this application, centrifugal force is not present. In our absorber the force is generated by a spring connected within the absorber either between the masses or between one mass and the base.
Another prior art absorber is shown in Desjardins et al U.S. Pat. No. 3,536,165 but it should be noted that this is not a bifilar vibration absorber, that it is a high friction and hence a high damping absorber and therefore a low amplification absorber so that it does not have the advantages of our bifilar vibration absorber.